Pursuant to the goal of making polyethylene terephthalate (PET) and other polyesters or co-polyesters, a great deal of patent literature is dedicated to describing the processes for preparing a dried carboxylic acid cake suitable as starting material. In general, these inventions describe specific mixing schemes with a purified terephthalic acid solid and liquid ethylene glycol. Additionally, there is substantial body of literature devoted to producing a purified terephthalic acid in the powder form that is suitable for use in producing PET and other polyesters or co-polyesters.
The objective of this invention is to describe a process by which the dried carboxylic acid cake suitable as a starting material for polyester or co-polyester production is obtained from a slurry or cake carboxylic acid product through the use of a counter current solvent wash zone. More specifically, the objective of this invention is to describe a process by which a dried terephthalic acid cake suitable as a starting material for polyester or co-polyester production is obtained from a slurry or cake terephthalic acid product through the use of a counter current solvent wash zone to reduce the amount of fresh solvent used in the process.
Usually, purified terephthalic acid solid is produced in a multi-step process wherein a crude terephthalic acid is produced. Liquid phase oxidation of p-xylene produces crude terephthalic acid. The crude terephthalic acid does not have sufficient quality for direct use as starting material in commercial PET. Instead, the crude terephthalic acid is usually refined to purified terephthalic acid solid.
Usually in terephthalic acid purification processes, the crude terephthalic acid is dissolved in water and hydrogenated for the purpose of converting 4-carboxybenzaldehyde to p-toluic acid, which is a more water-soluble derivative, and for the purpose of converting characteristically yellow compounds to colorless derivatives. Significant 4-carboxybenzaldehyde or p-toluic acid in the final purified terephthalic acid product is particularly detrimental to polymerization processes as each can act as a chain terminator during the condensation reaction between terephthalic acid and ethylene glycol in the production of PET. Typical purified terephthalic acid contains on a weight basis less than 25 parts per million (ppm) 4-carboxybenzaldehyde and less than 150 ppm p-toluic acid.
A number of other processes have been developed where a terephthalic acid suitable as starting material for commercial PET production without the use of hydrogenation. Typically, terephthalic production processes involve catalyzed oxidation of p-xylene in an acetic acid solvent followed by filtration and drying of the terephthalic acid.
Typically, terephthalic acid (TPA) produced via catalyzed oxidation of p-xylene in an acetic acid solvent produces a slurry or cake terephthalic acid product that contains residual catalyst (e.g cobalt, manganese, and bromine compounds). In a common method of producing a substantially dry TPA solid from a slurry or cake terephthalic acid product, the slurry or cake terephthalic acid product is filtered to separate a substantial amount of the acetic acid liquid from the TPA solids. Residual catalyst is usually separated from the slurry or cake terephthalic acid product by washing (rinsing) the wet cake with catalyst-free acetic acid, water or other solvent. The TPA solid is isolated by drying.
In the present invention, a novel process has been discovered resulting in less solvent used than currently employed processes. In the conventional approach toward producing terephthalic acid via catalyzed oxidation of p-xylene in an acetic acid solvent, a slurry or cake terephthalic acid product is filtered, washed, then dried to produce a terephthalic acid powder suitable as starting material for PET production.
In one embodiment of the present invention, the slurry or cake terephthalic acid product is filtered to produce a terephthalic acid cake with solvent and a TPA solvent mother liquor stream. The terephthalic acid cake with solvent is then washed (rinsed) with water to recover residual metal catalyst material and to produce a water-wet terephthalic acid cake and an TPA solvent/water by-product liquor. The water-wet terephthalic acid cake is then dried to produce a dried terephthalic acid cake suitable as starting material in a commercial PET process. In this embodiment of the invention at least one counter current wash is utilized. By utilizing a counter current solvent wash zone the amount of solvent used can be reduced substantially as compared to a process without counter current washing. In addition, by utilizing at least one counter current wash may result in reduction of equipment size and energy as compare to a TPA production process without a counter current wash.